1. Field of the Invention
The present invention relates generally to a writing instrument employing a strain gage transducer apparatus, and more particularly, to a writing instrument which transmits the motion of its pen tip through a reservoir and a housing to a flexible diaphragm which distorts in accordance with the force applied to, and the direction of motion of, the pen tip.
2. Description of the Prior Art
Character recognition systems for converting each character of a hand printed message, as it is being written, to a set of electrical signals which uniquely identify each character of the hand written message include a pen which produces the physical character and which develops an electrical signal representative of the direction being taken.
Heretofore such pens have typically included a ball point ink cartridge having a tip which is adapted to contact the writing medium and an opposed end which is directly attached to a flexible diaphragm. The diaphragm is mounted in a housing with its periphery being clamped to the housing in such a manner that the diaphragm is capable of distorting in response to force exerted on and the direction of motion of the tip. Strain gage transducers attached to a surface of the diaphragm exhibit characteristic electrical resistance changes corresponding to the distortion of the diaphragm and are connected in a bridge circuit which produces a resultant electrical signal that is representative of the force on and direction of motion of the tip. However, problems have been encountered in maintaining a tight bond around the entire periphery of the diaphragm and in many pens, portions of the periphery separate slightly from the housing. A disadvantage of such separation is that in operation the diaphragm tends to distort nonlinearly, such distortion resulting in a degradation of system performance. In addition, because the cartridge is mounted directly to the diaphragm, the pen is relatively insensitive to forces which produce only small distortions in the diaphragm. Another disadvantage is that such pens have included an external ink carrying reservoir which is not integral with the cartridge and is subject to accumulating air bubbles in the ink when the pen is turned upside down. This tends to cause fluctuations in the intensity of the characters written by the pen.
An example of such a pen may be found in U.S. Pat. No. 3,915,015, entitled "Strain Gauge Transducer System", by H. D. Crane, J. C. Taenzer and Gerry B. Andeen. Other examples of the prior art may be found in U.S. Pat. No. 3,145,367, entitled "Character Recognition Circuit", by H. D. Crane; U.S. Pat. No. 3,462,548, entitled "Combined Writing Device and Computer Input", by Robert M. Rinder; and in U.S. Pat. No. 3,906,444, entitled "Special Pen And System For Handwriting Recognition", by H. D. Crane and R. E. Savoie.